diff --git a/README.ko.md b/README.ko.md
index 33b60912..234d5a14 100644
--- a/README.ko.md
+++ b/README.ko.md
@@ -53,6 +53,10 @@ pip install "./vdsim-*.whl[plot]"
 vdsim-quickstart          # cwd 에 run.csv + run.png 생성
 ```
 
+깨끗한 환경에 설치할 것 — 최상위 `vdsim` 패키지를 제공하는 다른 배포판이 함께 설치되면
+그쪽이 import 를 가져간다. VDSim 은 이를 감지해 실제 로드된 파일 경로와 함께
+`CoreShadowedError` 를 던진다.
+
 측정 (clean conda, Python 3.11, Linux x86_64, 2026-06-25): **첫 결과까지 수 초** —
 pip install `[plot]` + `vdsim-quickstart` → `run.csv` + `run.png` **~6 s wall-clock**
 (cold; lab 네트워크, matplotlib wheel 포함). 재실행 ~1.5 s.
diff --git a/README.md b/README.md
index 56a9c009..f0d46349 100644
--- a/README.md
+++ b/README.md
@@ -55,6 +55,10 @@ pip install "./vdsim-*.whl[plot]"
 vdsim-quickstart          # writes run.csv + run.png in the current directory
 ```
 
+Install into a clean environment: another distribution also provides a top-level
+`vdsim` package, and if both are present that one wins the import. VDSim detects
+this and raises `CoreShadowedError` naming the file that was loaded.
+
 Measured on a clean conda env (Python 3.11, Linux x86_64, 2026-06-25): **first result in
 seconds** — pip install `[plot]` + `vdsim-quickstart` → `run.csv` + `run.png` in **~6 s
 wall-clock** (cold; lab network, matplotlib wheel included). Repeat quickstart ~1.5 s.
diff --git a/docs/VALIDATION.md b/docs/VALIDATION.md
index f7b4fdd7..331f7df9 100644
--- a/docs/VALIDATION.md
+++ b/docs/VALIDATION.md
@@ -52,7 +52,7 @@ tests:    505/505
 config:   cmake --preset validation && cmake --build --preset validation && ctest --preset validation
 presets:  CMakePresets.json@fb1f0c198a0201fcce2543a9f5b2242a255fb97a
 toolchain: cmake 3.31.10
-commit:   3a72f3c
+commit:   7b7947f
 date:     2026-09-22
 excluded: gui_v3_e2e (GUI v3 test group formally deferred -- VDSIM_BUILD_GUI_V3_TESTS=OFF)
 excluded: gui_v3_api_smoke (GUI v3 test group formally deferred -- VDSIM_BUILD_GUI_V3_TESTS=OFF)
diff --git a/python/CMakeLists.txt b/python/CMakeLists.txt
index 09be8a13..b32f5e2f 100644
--- a/python/CMakeLists.txt
+++ b/python/CMakeLists.txt
@@ -21,6 +21,7 @@ if(pybind11_FOUND)
     if(DEFINED SKBUILD)
         install(TARGETS vdsim_py LIBRARY DESTINATION ".")
         install(FILES
+            ${CMAKE_SOURCE_DIR}/python/vdsim_guard.py
             ${CMAKE_SOURCE_DIR}/python/vdsim_lab.py
             ${CMAKE_SOURCE_DIR}/python/vdsim_plant.py
             ${CMAKE_SOURCE_DIR}/python/vdsim_trace.py
diff --git a/python/vdsim_guard.py b/python/vdsim_guard.py
new file mode 100755
index 00000000..1efb491d
--- /dev/null
+++ b/python/vdsim_guard.py
@@ -0,0 +1,119 @@
+#!/usr/bin/env python3
+"""Single check that the name ``vdsim`` resolves to VDSim's compiled core.
+
+Another distribution on PyPI ships a top-level ``vdsim/`` *package*.  Installed
+into the same environment as our wheel it wins the import over our
+``vdsim.cpython-*.so`` -- pip neither warns nor overwrites, so the failure
+surfaces much later as an ``AttributeError`` on an unrelated line.  The modules
+that need the core therefore call :func:`load_core` instead of ``import
+vdsim``; the decision itself lives here once so the modules cannot drift.
+
+The check is structural on purpose: it asks whether the loaded module is a
+compiled extension exposing the core API, never which distribution shipped it.
+"""
+from __future__ import annotations
+
+import importlib
+import importlib.machinery
+from pathlib import Path
+
+__all__ = ["CoreShadowedError", "check_core", "load_core", "diagnose"]
+
+
+class CoreShadowedError(ImportError):
+    """``vdsim`` resolved to something that is not VDSim's compiled core."""
+
+
+#: Symbols every build of the core exposes (pybind11 ``cosim/bindings.cpp``).
+#: Checked as a set so a foreign module cannot pass by coincidence on one name.
+EXPECTED_SYMBOLS = ("SimSession", "VehicleParams", "TireParams", "make_sim_session")
+
+
+def diagnose(mod):
+    """Return ``None`` if *mod* is the compiled core, else a one-line reason.
+
+    :param mod: the module object bound to the name ``vdsim``.
+    :returns: ``None`` when the module passes both checks, otherwise a short
+              phrase naming which property failed.
+    """
+    origin = getattr(mod, "__file__", None)
+    if origin is None:
+        return "it has no __file__ (a namespace package, not an extension module)"
+    if not any(str(origin).endswith(s)
+               for s in importlib.machinery.EXTENSION_SUFFIXES):
+        return "it is Python source, not a compiled extension module"
+    missing = [s for s in EXPECTED_SYMBOLS if not hasattr(mod, s)]
+    if missing:
+        return "it is missing core symbols: %s" % ", ".join(
+            "vdsim." + s for s in missing)
+    return None
+
+
+def _message(mod, reason):
+    origin = getattr(mod, "__file__", None) or "<no __file__>"
+    return (
+        "'import vdsim' loaded %s -- %s.\n"
+        "Cause: another installed distribution provides a top-level 'vdsim' "
+        "package, which shadows VDSim's vdsim.cpython-*.so on sys.path "
+        "(pip does not warn about this).\n"
+        "Fix: uninstall the shadowing distribution "
+        "(e.g. `pip uninstall vehicle-dynamics-sim`) or install VDSim into "
+        "its own virtual environment." % (origin, reason)
+    )
+
+
+def check_core(mod):
+    """Return *mod* if it is the compiled core, else raise.
+
+    :param mod: the module object bound to the name ``vdsim``.
+    :raises CoreShadowedError: when :func:`diagnose` reports a mismatch.
+    """
+    reason = diagnose(mod)
+    if reason is not None:
+        raise CoreShadowedError(_message(mod, reason))
+    return mod
+
+
+def load_core(fallback_paths=()):
+    """Import ``vdsim`` and return it only if it is the compiled core.
+
+    *fallback_paths* are development build directories tried, in order, when
+    the ambient import is missing **or** shadowed; this keeps a repo checkout
+    working even in an environment that has the shadowing package installed.
+
+    :param fallback_paths: directories prepended to ``sys.path`` on retry.
+    :returns: the compiled ``vdsim`` extension module.
+    :raises CoreShadowedError: the name resolves to a foreign module and no
+            fallback path yields the core.
+    :raises ImportError: ``vdsim`` is not importable at all.
+    """
+    import sys
+
+    first_error = None
+    try:
+        mod = importlib.import_module("vdsim")
+    except ImportError as exc:
+        mod, first_error = None, exc
+    if mod is not None and diagnose(mod) is None:
+        return mod
+
+    for raw in fallback_paths:
+        path = str(raw)
+        if not Path(path).is_dir():
+            continue
+        while path in sys.path:
+            sys.path.remove(path)
+        sys.path.insert(0, path)
+        sys.modules.pop("vdsim", None)
+        importlib.invalidate_caches()
+        try:
+            candidate = importlib.import_module("vdsim")
+        except ImportError:
+            continue
+        if diagnose(candidate) is None:
+            return candidate
+        mod = candidate
+
+    if mod is None:
+        raise first_error
+    return check_core(mod)
diff --git a/python/vdsim_lab.py b/python/vdsim_lab.py
index 42d09d2c..7825d16b 100644
--- a/python/vdsim_lab.py
+++ b/python/vdsim_lab.py
@@ -20,11 +20,9 @@
 from pathlib import Path
 
 REPO = Path(__file__).resolve().parent.parent
-try:
-    import vdsim
-except ImportError:
-    sys.path.insert(0, str(REPO / "build" / "python"))
-    import vdsim
+from vdsim_guard import load_core  # noqa: E402  (shadowed-core check, one place)
+
+vdsim = load_core([REPO / "build" / "python"])
 
 
 def _conf_root():
diff --git a/python/vdsim_plant.py b/python/vdsim_plant.py
index 82afb3d6..70e12424 100644
--- a/python/vdsim_plant.py
+++ b/python/vdsim_plant.py
@@ -29,11 +29,9 @@
 from pathlib import Path
 
 REPO = Path(__file__).resolve().parent.parent
-try:
-    import vdsim
-except ImportError:
-    sys.path.insert(0, str(REPO / "build" / "python"))
-    import vdsim
+from vdsim_guard import load_core  # noqa: E402  (shadowed-core check, one place)
+
+vdsim = load_core([REPO / "build" / "python"])
 
 def _conf_root():
     for c in (REPO / "configs", Path.cwd() / "configs",
diff --git a/python/vdsim_rl/__init__.py b/python/vdsim_rl/__init__.py
index bd8c6862..e67fef08 100644
--- a/python/vdsim_rl/__init__.py
+++ b/python/vdsim_rl/__init__.py
@@ -24,7 +24,9 @@
 
 import numpy as np
 
-import vdsim
+from vdsim_guard import load_core
+
+vdsim = load_core()
 
 __all__ = [
     "EnvConfig", "VDSimVecEnv", "VDSimEnv", "make_sb3_vec_env",
diff --git a/tests/scripts/test_experiment_api.py b/tests/scripts/test_experiment_api.py
index 274a5cdb..9496e411 100644
--- a/tests/scripts/test_experiment_api.py
+++ b/tests/scripts/test_experiment_api.py
@@ -147,6 +147,74 @@ def test_ref_point_position():
     assert sim_u._ref == [1.0, 0.5]
 
 
+# --- Q21: the name `vdsim` must resolve to the compiled core -----------------
+# An unrelated distribution ships a top-level `vdsim/` package; installed beside
+# our wheel it wins the import silently.  vdsim_guard makes that an error.
+
+def _shadow_probe(body, shadow_dir, extra_path=()):
+    """Run *body* in a child whose sys.path starts with a fake `vdsim` package."""
+    import os
+    import subprocess
+    env = dict(os.environ)
+    env["PYTHONPATH"] = os.pathsep.join(
+        [str(shadow_dir), str(REPO / "python")] + [str(p) for p in extra_path])
+    return subprocess.run([sys.executable, "-c", body], env=env,
+                          capture_output=True, text=True)
+
+
+def _make_shadow(td):
+    pkg = Path(td) / "vdsim"
+    pkg.mkdir()
+    (pkg / "__init__.py").write_text("VERSION = 'foreign package'\n")
+    return Path(td)
+
+
+def test_shadowed_core_is_rejected():
+    body = ("import sys, vdsim_guard\n"
+            "try:\n"
+            "    vdsim_guard.load_core([])\n"
+            "except vdsim_guard.CoreShadowedError as exc:\n"
+            "    print(exc); sys.exit(0)\n"
+            "sys.exit('no error raised')\n")
+    with tempfile.TemporaryDirectory() as td:
+        shadow = _make_shadow(td)
+        r = _shadow_probe(body, shadow)
+        assert r.returncode == 0, f"shadowed vdsim was accepted: {r.stdout}{r.stderr}"
+        msg = r.stdout
+        assert str(shadow / "vdsim") in msg, f"message must name the real load path: {msg}"
+        assert "Cause:" in msg and "Fix:" in msg, f"message needs cause + remedy: {msg}"
+
+
+def test_shadowed_core_recovers_from_build_dir():
+    """A dev checkout still works: the explicit build path wins over the shadow."""
+    import os
+    build = Path(os.environ.get("VDSIM_BUILD_DIR", REPO / "build")) / "python"
+    if not build.is_dir():
+        build = REPO / "build" / "python"
+    assert build.is_dir(), f"no compiled core to fall back to ({build})"
+    body = ("import sys, vdsim_guard\n"
+            "m = vdsim_guard.load_core([sys.argv[1]])\n"
+            "assert hasattr(m, 'SimSession'), m\n"
+            "print(m.__file__)\n")
+    with tempfile.TemporaryDirectory() as td:
+        shadow = _make_shadow(td)
+        import os as _os
+        import subprocess
+        env = dict(_os.environ)
+        env["PYTHONPATH"] = _os.pathsep.join([str(shadow), str(REPO / "python")])
+        r = subprocess.run([sys.executable, "-c", body, str(build)],
+                           env=env, capture_output=True, text=True)
+        assert r.returncode == 0, f"fallback failed: {r.stdout}{r.stderr}"
+        assert str(shadow) not in r.stdout, r.stdout
+
+
+def test_real_core_passes_the_check():
+    import vdsim
+    import vdsim_guard
+    assert vdsim_guard.diagnose(vdsim) is None, vdsim.__file__
+    assert vdsim_guard.check_core(vdsim) is vdsim
+
+
 if __name__ == "__main__":
     test_throttle_then_brake()
     test_step_steer_yaws()
@@ -159,4 +227,7 @@ def test_ref_point_position():
     test_register_metric()
     test_plot_comparison()
     test_ref_point_position()
+    test_shadowed_core_is_rejected()
+    test_shadowed_core_recovers_from_build_dir()
+    test_real_core_passes_the_check()
     print("OK test_experiment_api")